Characteristic fast H− ion conduction in oxygen-substituted lanthanum hydride

Fast ionic conductors have considerable potential to enable technological development for energy storage and conversion. Hydride (H − ) ions are a unique species because of their natural abundance, light mass, and large polarizability. Herein, we investigate characteristic H − conduction, i.e., fast ionic conduction controlled by a pre-exponential factor. Oxygen-doped LaH 3 (LaH 3 −2 x O x ) has an optimum ionic conductivity of 2.6 × 10 −2 S cm −1 , which to the best of our knowledge is the highest H − conductivity reported to date at intermediate temperatures. With increasing oxygen content, the relatively high activation energy remains unchanged, whereas the pre-exponential factor decreases dramatically. This extraordinarily large pre-exponential factor is explained by introducing temperature-dependent enthalpy, derived from H − trapped by lanthanum ions bonded to oxygen ions. Consequently, light mass and large polarizability of H − , and the framework comprising densely packed H − in LaH 3 − 2 x O x are crucial factors that impose significant temperature dependence on the potential energy and implement characteristic fast H − conduction. Hydride ions are promising for energy storage since they are abundant, lightweight, and highly mobile, but ionic conductivity should be improved. Here the authors achieve fast hydride ion conductivity in a mixed-anion compound by tuning oxygen content.